This invention relates to a process and apparatus for removal of silica from water, especially circulating water in a cooling tower.
Naturally occurring silica dissolved or suspended in water circulated in evaporative cooling towers tends to come out of solution when the water is subjected to evaporation concentration. Because silica solubility is inversely temperature dependent to that of calcium carbonate, silica will become supersaturated in the hot portions of circulating water loops in evaporative cooling towers. Calcium carbonate is less solutble in the hot portions of the circulating water loops. Silica may co-deposit on the crystal matrices of either scale or corrosion products that occur in the circulating water loops. Co-deposits of silica may occur at silica concentrations that are less than the saturation concentration for amorphous silica alone. Any of these forms of deposition can restrict water flow and heat exchange.
Chemical treatment using chelates or sequestrants, water softening agents, or pH control have been used to prevent deposition and scale formation with varying degrees of success. For example, dissolved and colloidal silica is commonly removed from water by increasing the pH above 10.5 to convert the silica from acid to predominantly silicate ions, then adding a source of magnesium such as magnesium sulfate or magnesium chloride, after which a source of hydroxide ion is added to precipitate magnesium hydroxide. If the hydroxide ion source is lime, calcium carbonate is also formed. Iron and organic polyelectrolytes are also commonly added to assist settling of floc. Large quantities of sludge and dissolved solids are left in the water being treated. In general, use of chemical treatments greatly increases the amounts of water consumption and creates undesirable waste streams.
Chemically preventing deposition of calcium carbonate in heat exchanges has been used to reduce co-deposition of silica in the heat exchangers. However, in the cooler parts of the system such as on the tower fill, deposition of silica is not prevented by preventing deposition of calcium carbonate.
Numerous patents and other publications describe attempts to remove silica from water and, conversely, attempts to keep the silica in solution as the water is heated, cooled or evaporated or some combination thereof, to prevent deposition of the silica on cooling tower surfaces or other surfaces. Some of these include: U.S. Pat. No. 4,276,180 which discloses removal of silica from waste-water streams by passing the water over activated alumina which is regenerated for reuse and which generates a large waste stream; and U.S. Pat. No. 4,370,858 which discloses use of in situ generated silica particles or pre-treated particles of metal to remove silica from hot water in a solids contact clarification reaction downstream of an energy recovery unit.
There is still a need for ways of preventing deposition of silica on evaporative cooling tower surfaces, particularly where it diminishes heat transfer and water flow.
An object of this invention is to provide a method and apparatus for preventing silica deposition on heat-exchange surfaces contacted with aqueous silica solutions.
Another object of this invention to provide a method of removing silica from water by providing a surface for preferential deposition of the silica.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentality""s and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, there has been invented a process in which small amorphous nucleation site material particles are used to provide a relatively large surface area upon which silica will preferentially adsorb, thereby preventing or substantially reducing scaling caused by deposition of silica on evaporative cooling tower components, especially heat exchange surfaces. The silica spheres are contacted by the cooling tower water in a sidestream reactor, then separated using gravity separation, microfiltration, vacuum filtration, centrifugal separation, or other suitable separation technology. Cooling tower modifications for implementing the invention process have been designed.